In conversation with my family, I realised that they don't know what a 12 string guitar is and what makes it special. On this basis, I imagine that most of my readership doesn't know either, so I have decided to write a few words of explanation.
A regular six string guitar with standard tuning is tuned (from bottom to top) E - A - D - G - B - E. Apart from the G-B gap, all the strings are tuned in fourths. Questions have been asked on the Internet as to why the intervals are fourths, except for one; the explanation seems to be that this tuning was settled on after decades of trials, as it allows for the easy formation of chords. Tuning the top two strings a semitone higher (thus preserving intervals of fourths over the entire guitar) apparently makes for very difficult chord shapes. A bass guitar has only four strings: these are tuned as the same as the bottom four strings on a guitar but an octave lower.
Since the early 1960s, people have experimented with alternative tunings, such as 'DADGAD', dropped D and open G (Keith Richards). Robert Fripp developed a 'new standard tuning' which is fifths. Joni Mitchell rarely played in standard tuning (as a side effect of the polio from which she suffered as a child) and so developed special tunings: almost every one of her songs has a different tuning.
A mandolin has eight strings: two 'courses' of four strings, which are tuned (from bottom to top) G - D - A - E, which are intervals of fifths. Each string is doubled, which gives the mandolin its special sound; frequently the two strings for each note are slightly out of tune, which gives a chorus effect. One of the bazoukis which I was shown in Rodos has six strings, two courses of three, tuned A - D - A.
After that lengthy introduction, we can now turn to the twelve string guitar. Like the mandolin and the bazouki, this guitar has its strings doubled (that's why there are twelve strings and not six). But instead of tuning the strings in unison (mandolin and bazouki), the bottom four strings are tuned in octaves (the top two are in unison). This is what gives the twelve string its characteristic sound, and is why I wrote yesterday that I will have to learn how to play single note parts on the lower strings.
Where can the twelve string be heard? As a child of the 60s, I instinctively think of the "A Hard Day's Night" album, where George Harrison can clearly be heard playing a twelve string. The nascent Byrds saw the film and heard the music; they turned the twelve string into the dominant sound of their earlier songs. The first line up of Genesis used acoustic twelve strings. Apparently Tom Petty uses a twelve string guitar but I am not familiar with his work. Fairport used a twelve string occasionally, played by Simon Nicol: the opening riff on "Come all ye" and "Run, Johnny, Run" spring to mind. In fact, Simon used to claim that the reason that he was asked to join Fairport in 1967 was that he had a twelve string guitar!